The present invention relates generally to illumination control and more specifically to increasing the manufacturing yield in an optical system used for reading bar codes and position reference marks, by adjusting the intensity of the illumination source with pulse width modulation (PWM).
Jukebox storage devices (also called mass storage auto-changers) typically contain a number of data cartridges. These data cartridges are typically stored in cartridge magazines. The magazines typically contain a number of slots or spaces where the data cartridges can be stored. The jukebox may contain a plurality of magazines. Typically the magazines are removable from the jukebox storage device for easier access to the magazines so the magazines can be loaded with data cartridges. Typically each data cartridge is labeled such that the jukebox storage device can identify and keep track of the location of each data cartridge. Typically the data cartridge is labeled with a barcode and the jukebox storage device contains an optical system that can read bar codes. The optical system is typically referred to as a barcode reader, however the optical system typically is used for tasks in addition to reading barcodes. For example the jukebox storage device also needs to be able to determine if a magazine is loaded into the jukebox and if a slot or space in the magazine contains a data cartridge. Typically the magazines are labeled such that the jukebox storage device can use the barcode reader to detect if a magazine is present. The barcode reader can also be used to determine the location of the magazine. The jukebox storage device also typically uses the barcode reader to determine if a slot or space in the magazine contains a data cartridge. Because the barcode reader in the jukebox storage device is used for a number of different tasks (i.e. cartridge detection, magazine detection, magazine location, and cartridge identification) the intensity of light detected by the barcode reader from a given target needs to be well defined.
Typically the barcode reader (see FIG. 1) contained within a jukebox storage device uses a charged coupled device (CCD) (102) to measure the reflected light, a light emitting diode (LED) (110) as the illumination source and two lenses, one lens (108) to focus the light from the LED (110) onto the target (106) and one lens (104) to focus the reflected light from the target (106) onto the CCD (102). Each of the elements of the barcode reader typically has some manufacturing tolerance that creates differences in the performance of the total barcode reader. For example two LED""s created by the same manufacturing process typically have a different light output for the same electrical input. Alignment of the LED to its focusing lens and the alignment of the CCD to its focusing lens can also affect the amount of light detected by the CCD for a given target. Because of the differences in the individual part tolerances and the alignment tolerances of the optical system the total variation of light detected at the CCD for a given target between two different barcode readers can be as large as 50%. This large variation in light detected between two different barcode readers makes it difficult for barcode readers to be able to fulfill all of the required tasks. For example, FIG. 2 shows a typical target used to determine the presence of a magazine in a jukebox storage device. FIG. 3 shows the intensity of the detected light (302) from the target as the barcode reader sweeps across the target. Typically the intensity of the detected light (302) must start above and then fall below a threshold (304) before the jukebox storage device considers that a target has been successfully detected. FIG. 4 shows the intensity of the detected light from a target when the light that makes it through the system is too low (404) and when the light that makes it through the system is too high (402). When the light that makes it through the system is too high (402) the intensity never falls below the threshold (406) and no target is detected. When the light that makes it through the system is too low (404) the intensity never gets above the threshold (406) and no target is detected.
One way to overcome this variation in barcode readers is to measure the light detected by the barcode readers with a given target and keep the barcode readers that meet a narrow tolerance of detected light. Because the required tolerance on the measured reflected light is small and the variation in the measured reflected light is large, the yield of usable barcode readers is small.
There is a need for an improvement in the yield of optical systems used as barcode readers in jukebox storage devices.
A method and apparatus that allows the adjustment of the intensity of the illumination source in a barcode reader, using the barcode reader as the measuring device, to give a predetermined amount of light from a known target. The adjustment of the intensity of the illumination source in the barcode reader compensates for the manufacturing tolerances of the barcode reader.